The invention relates to a method for measuring forces exerted on a piece in a given direction, and in the presence of an external pressure which applies a force on said piece that is large compared with the forces to be measured.
A preferred application of the invention lies in measuring compression or tension forces exerted on a member such as a downhole tool or a drill string, e.g. when the member is put into place in a oil well or when it is extracted from such a well. The very high pressures existing at the bottom of a well then apply force on the member in question that is considerably greater than the force to be measured.
French patent application FR 99 11948 filed on Sep. 24, 1999 in the name of the Applicant describes a method of measuring forces F exerted on a piece in a first given direction and in the presence of an external pressure Pe, the method being designed to be practically insensitive to the forces generated by said pressure. In that method, the deformation of a wall is measured in an axial direction parallel to the first direction and at at least two points of different wall thicknesses in a second direction forming a non-zero angle with the first direction in order to compensate for the pressure effect.
Nevertheless, that method requires a plurality of strain gauges to be used in order to perform the various measurements. It also requires a piece to be used that is specially designed for the method, having walls of unequal thickness. Implementing such a method thus requires considerable instrumentation and its cost becomes large.
An object of the invention is to provide a method of measuring a force F exerted on a piece in a given direction in the presence of external temperature and pressure, said method being designed to compensate the forces generated by said temperature and said pressure.
According to the invention, this result is obtained by means of a method of measuring a force F exerted on a piece in a given direction, in the presence of an external pressure Pe and a temperature T, in which:
the response Rcap of a deformation sensor to the total force exerted on the piece is measured;
said external pressure Pe and said temperature T are determined; and
the force F exerted on the piece in said given direction is determined by a relationship between the response of said deformation sensor (Rcap) and a polynomial function of degree n of the pressure Pe [fn(Pe)], said relationship being such that:
F=K.Rcapxe2x88x92fn(Pe) 
where K is a constant determined by calibration of the sensor.
This method is particularly advantageous since it makes it possible to use pressure and deformation sensors known in the state of the art, thus making the measurements simple to perform while being very reliable. Furthermore, this calculation model makes it possible to obtain the value of the force F exerted on a piece in a given direction without using an additional deformation sensor and even when the external pressure Pe is large.
In an implementation of the invention, the force F exerted on the piece is determined using the following relationship:   F  =            K      ·              R        cap              -                  ∑                  i          =          0                n            ⁢                                    A            i                    ⁡                      (            T            )                          ·                  Pe          i                    
the coefficients Ai(T) being a function of temperature T and being determined experimentally.
In an advantageous implementation of the invention, the coefficients Ai(T) are polynomial functions of temperature and of order m such that:             A      i        ⁡          (      T      )        =            ∑              j        =        0            m        ⁢                  B        ij            ·              T        j            
where the coefficients Bij are determined experimentally.
Advantageously, the function fn(Pe) is a second-order polynomial function of pressure Pe (n=2).
In an advantageous implementation of the invention, the coefficients Ai(T) are second-order polynomial functions of temperature (m=2).
This simplification of the polynomial functions fn(Pe) and Ai(T) into second order polynomial functions (n=m=2) makes it possible to determine the force F accurately while minimizing calculations for compensating the effects of pressure and of temperature.
In an advantageous implementation of the invention, the coefficients A0(T) and A1(T) are assumed to be constant with temperature and the coefficient A2(T) is assumed to be zero.
This implementation makes it possible to simplify determining the coefficients Bij to a great extent while conserving very acceptable accuracy for the measurements.